MOND - a general discussion

[Nereid]

Some of dgruss23's posts, in the No Dark Matter thread, provide good material on MOND (MOdified Newtonian Dynamics), a shorthand for a set of ideas, hypotheses, and nascent theories that have arisen from a conjecture by Milgrom, in 1983 {link/ref to be added}.

In particular, #10, , #17, #20, #25, and #28 (and the various links therein).

There is also a good (2002) review paper, by Sanders and McGaugh.

As a discussion of MOND is (largely) OT for the thread in which dgruss23 posted, I have started a new thread, to enable a more general discussion.

Here are some questions I have about MOND, per some of the papers in the links provided by dgruss23. Can any BAUT member reading this point to where answers may be found?

1) In Scarpa (2003), an H₀ of 50 is assumed - how do the various plots and conclusions of this paper change if 72 is used instead?

2) in the Sanders and McGaugh 2002 review paper, discussion of MOND compatibility with lensing observational data is limited to strong lensing (and the paper's claim is, crudely, that if there is strong lensing, then the acceleration regime must be non-MONDian). How does MOND handle weak lensing?

3) ditto, for weak lensing signals in general (e.g. the integrated galaxy weak lensing signal found by SDSS)?

4) What is the current status of the 'no-go' challenges to the various classes of super-MOND theories, which incorporate (modified) GR, wrt gravitational lensing? (this is covered in section 5.4 of the review paper, but it seems things were very tentative in 2002).

5) Has anyone tried fitting the rotation curves of the Milky Way, M31, LMC, SMC, IC5152, and M81 using MOND? (none of these galaxies appears in any of the McGaugh lists).

6) How does MOND fare when it is used to model galaxy collisions? (such as those done by the Toomre brothers, modelling the Antennae)

7) How well does MOND produce tidal tails and streams? (such as those of globular clusters and dwarf satellite galaxies being stripped, by the MW)

[dgruss23]

Some of dgruss23's posts, in the No Dark Matter thread, provide good material on MOND (MOdified Newtonian Dynamics), a shorthand for a set of ideas, hypotheses, and nascent theories that have arisen from a conjecture by Milgrom, in 1983 {link/ref to be added}.

In particular, #10, , #17, #20, #25, and #28 (and the various links therein).

There is also a good (2002) review paper, by Sanders and McGaugh.

As a discussion of MOND is (largely) OT for the thread in which dgruss23 posted, I have started a new thread, to enable a more general discussion.

First, let me say thanks for breaking this off. MOND is one of the most seriously studied ATM ideas and a thread devoted to it is worth having.

Second, I'm afraid the discussion we were having on the other thread got blown way beyond my intent. Correct me if I'm wrong, but I believe our disagreement boils down to this: You seem to hold the view that MOND is largely irrelevant - that MOND might be a handy formulation for fitting rotation curves but beyond that is of little value - since it is not a cosmological theory on its own.

My view of MOND is that it has already empirically established a fundamental fact about gravitational dynamics - that being the recognition that when applying Newtonian dynamics mass discrepancies appear in the low acceleration regime. I think our disagreement is more about this - I see that empirical reality as very important to the larger DM issue whereas you seem to be implying it is not important. This is a much more fundamental disagreement I have with your comments than the quibbling over MOND right/wrong and in what regimes.

Now let me also say that I am still examining the MOND literature and have been for quite a while. So while I find MOND interesting and will discuss my understanding of it, I do not wish to be seen as a "MONDian". My "defense" of MOND in so much as I may do so is more in regards to clearing up misunderstandings about it and in regards to my belief that there is some importance to what MOND researchers have found that will impact revisions to our models. In other words, I feel the literature has shown that "right" or "wrong", MOND is telling us something about the nature of gravity that theory must be able to account for.

I don't have time right now to examine all your questions in appropriate detail, but I'll provide a few additional links.

Here are some questions I have about MOND, per some of the papers in the links provided by dgruss23. Can any BAUT member reading this point to where answers may be found?

1) In Scarpa (2003), an H₀ of 50 is assumed - how do the various plots and conclusions of this paper change if 72 is used instead?

MOND fits can be sensitive to the distance scale used. I'll have to look into this. That is something you might do - if you really want to know.

2) in the Sanders and McGaugh 2002 review paper, discussion of MOND compatibility with lensing observational data is limited to strong lensing (and the paper's claim is, crudely, that if there is strong lensing, then the acceleration regime must be non-MONDian). How does MOND handle weak lensing?

3) ditto, for weak lensing signals in general (e.g. the integrated galaxy weak lensing signal found by SDSS)?

4) What is the current status of the 'no-go' challenges to the various classes of super-MOND theories, which incorporate (modified) GR, wrt gravitational lensing? (this is covered in section 5.4 of the review paper, but it seems things were very tentative in 2002).

With all these, I'll have to look into it - again, you could too. It should be relevant to your concerns given your position in your DM thread.

5) Has anyone tried fitting the rotation curves of the Milky Way, M31, LMC, SMC, IC5152, and M81 using MOND? (none of these galaxies appears in any of the McGaugh lists).

You would probably need to ask McGaugh why those have not been modeled with MOND. In the case of the MW, ther may be too much uncertainty in the surface brightness profile to perform a MOND fit (because we are inside the galaxy. A similar problem may arise with M-31,LMC, and SMC because of their large angular dimensions. AS for IC 5152 - is there something so special about the galaxy that we should have expected MOND researchers to have tackled it? Perhaps with M-81 its interaction with M-82 and NGC 3077 has created a problem for modeling it? Again, McGaugh could no doubt tell you.

6) How does MOND fare when it is used to model galaxy collisions? (such as those done by the Toomre brothers, modelling the Antennae)

I don't think it has been.

7) How well does MOND produce tidal tails and streams? (such as those of globular clusters and dwarf satellite galaxies being stripped, by the MW)

Here is an example and here is another.

Finally a few unresolved questions I have which I would like answers to.

Could you explain what you meant by this:

Then, as we all know that it is inconsistent with relativity, and that relativity has passed all its tests, with flying colours, why use a theory (the "ND" part implies a theory) that is known to be "wrong"?

If mainstream researchers use Newtonian dynamics, why do you think it is a waste to work on a modification of that dynamic?

and I'd also like an answer to this question:

Or are you disputing that MOND researchers have shown there is an acceleration scale to the mass disrepancy problem?

And I have also linked to papers that show MOND to be consistent with the dynamics of globular clusters, elliptical galaxies, and the research shows MOND reduces the mass discrepancy in galaxy clusters from ~ 10 to only ~2-3. So are you still taking the position that this is all MOND does?:

What MOND does do, and well, is fit the rotation curves of (some) spiral galaxies and (some) dwarf galaxies.

[Jerry]

5) Has anyone tried fitting the rotation curves of the Milky Way, M31, LMC, SMC, IC5152, and M81 using MOND? (none of these galaxies appears in any of the McGaugh lists).

Too much proper motion in the milky way, likewise the rotations of very close galaxies is difficult to model because of proper motions. A candidate for MOND studies must be viewed at an angle that allows both surface brightness measurements and Doppler velocities within the galactic plane. These criteria should not normally introduce a selection bias.

[turbo-1]

MOND is an ad-hoc correction of Newtonian dynamics that correctly predicted the behavior of low surface brightness galaxies at least a decade in advance of their observation. MOND may not have global applicability, but may instead be a marker of just how and to what extent Newtonian dynamics falls apart on galactic scales. This begs for epistemology. At what point could such an error have crept in? Are our observations in error? Are our assumptions about gravitational attraction in error? Could the fabric of space-time be polarized on large scales such that gravitational forces are not exactly the same in every domain? When you put all this on the table with the fact that clusters are far more gravitationally-bound than we expect given the calculated masses, it is hard to ignore that elephant in the room -- gravity on large scales does not act as we expect from Newton and GR. Since gravity is the dominant force on these scales, it seems rather cavalier to say that Newton got it right, and Einstein refined that, and then press on. Neither of them managed to get beyond a mathematical model of the effects of gravitation, though Einstein struggled for the rest of his life to lay out a mechanical model explaining how gravitation and inertia arise from matter's interaction with space-time. MOND has quantified the error of the Standard Model on galactic scales. If it serves no other purpose, it has served us well, indeed.

[folkhemmet]

I am not a cosmologist, but I think it is fair to say that MOND will not go away until dark matter particles are detected. It should be interesting to see how MOND adherents react at the time, if such a time comes, DM particles are discovered. There has been some recent chatter in the cosmology/particle physics community which predicts a DM detection within the next five to ten years, as dark matter direct detection experiments are on the verge of being able to probe a substantial portion of the "dark matter parameter space", and/or the LHC may produce exotic heretofore unseen particles.

If DM is found it will be as much of a fundamental step forward in our understanding of the matter as the discovery of new chemical elements of the periodic table. Some may argue that finding DM will be even more of a breakthrough since it makes up a greater percentage of all matter than do the elements. On the other hand, perhaps particle dark matter will not show up and we will be forced to revise our ideas about gravity. Either way excites me-- since either a definitive detection of particle dark matter or a refutation of particle dark matter will constitute major progress in terms of understanding the Universe.

[RussT]

MOND is an ad-hoc correction of Newtonian dynamics that correctly predicted the behavior of low surface brightness galaxies at least a decade in advance of their observation. MOND may not have global applicability, but may instead be a marker of just how and to what extent Newtonian dynamics falls apart on galactic scales. This begs for epistemology. At what point could such an error have crept in? Are our observations in error? Are our assumptions about gravitational attraction in error? Could the fabric of space-time be polarized on large scales such that gravitational forces are not exactly the same in every domain? When you put all this on the table with the fact that clusters are far more gravitationally-bound than we expect given the calculated masses, it is hard to ignore that elephant in the room -- gravity on large scales does not act as we expect from Newton and GR. Since gravity is the dominant force on these scales, it seems rather cavalier to say that Newton got it right, and Einstein refined that, and then press on. Neither of them managed to get beyond a mathematical model of the effects of gravitation, though Einstein struggled for the rest of his life to lay out a mechanical model explaining how gravitation and inertia arise from matter's interaction with space-time. MOND has quantified the error of the Standard Model on galactic scales. If it serves no other purpose, it has served us well, indeed.

Turbo-1, a very insightful analysis

[Neither of them managed to get beyond a mathematical model of the effects of gravitation,]

While Newton was certainly about effects, Einstein went beyond 'just effects'.

Although the warping of space/time is an effect, and explains gravity dealing with Baryonic Mass, in a more thorough and useful way, the ultimate warping of space/time; Black Holes and Singularities is cause and effect!


[though Einstein struggled for the rest of his life to lay out a mechanical model explaining how gravitation and inertia arise from matter's interaction with space-time.]

And I now have no doubt what-so-ever, after reading a String/"M" theorists paper (which I am not going to share at this time!), that Einstein, had he known of Massive Black Holes (he definitely would have figured out that they are different) and String/"M" Theory, would have easily figured out the 'rest of the gravity' story!

[Thanatos]

The strongest argument in favor of dark matter is the large scale structure of the universe. Without dark matter, it is difficult to explain galactic clustering. It is also difficult to explain the dark matter based numerical simulations that so elegantly predict the large scale filamentary distribution of galaxies in the universe:
http://en.wikipedia.org/wiki/Dark_ma...ture_formation

[turbo-1]

The strongest argument in favor of dark matter is the large scale structure of the universe. Without dark matter, it is difficult to explain galactic clustering. It is also difficult to explain the dark matter based numerical simulations that so elegantly predict the large scale filamentary distribution of galaxies in the universe:
http://en.wikipedia.org/wiki/Dark_ma...ture_formation

This is not an argument for dark matter. It is an argument for stronger-than-expected gravitational attraction in some domains, and the reason for this is unknown, at present. Unfortunately, the mainstream was unwilling or unable to question our assumptions regarding gravitation, and resorted instead to the invention of yet another entity to explain the gap between our expectations and reality. That so many people have bought into the Dark Matter idea instead of questioning our gravitational model is a sad commentary on the lack of epistemology in cosmology today. Despite its inability to interact with baryonic matter, this wonderful entity somehow obediently arranges itself around galaxies, in clusters, and on supercluster scales in just the right ways to make the standard cosmology conform to observations in each instance. How convenient. Let's back up to the one thing that we know - gravity does not behave as we expected on large scales - and start from there, asking why that is.

Einstein was looking for an "ether" to fix the shortcomings of GR - a dynamical entity that defines the structure of space-time and is conditioned by the matter embedded in it. He was on the path that the mainstream should be following now. We already know that the vacuum is not empty, but is a teeming sea of virtual particle pairs popping in and out of existence. Is that enough of a hint?

It is not enough to think of gravitation kinematically, with bodies interacting against a passive space-time field. Gravitation is dynamic. The field that confers gravitational attraction on matter is necessarily polarized and densified by that interaction, which means gravitational attraction varies with field strength, not just with mass and physical separation. Dark matter is just a place-holder - a bookkeeping trick to keep things working until we finally figure this out. We should never make the mistake of thinking that DM is real. That way lies decades of stagnation.

[snarkophilus]

Despite its inability to interact with baryonic matter, this wonderful entity somehow obediently arranges itself around galaxies, in clusters, and on supercluster scales in just the right ways to make the standard cosmology conform to observations in each instance.

Well, no... they say it does interact with baryonic matter. Just not through photons, but through gravity instead.

We should never make the mistake of thinking that DM is real. That way lies decades of stagnation.

I'm undecided, and awaiting the next installment in Nereid's thread in the Q&A section. I'm not convinced for or against at the moment, but it appears that there is some very strong observational evidence for it. I'm the sort of person who is predisposed to a MOND-like solution to the problems that dark matter tries to solve, but there's a lot of evidence for dark matter. If I can be shown that DM theory allows us to make predictions, I'll be close to sold on it.

There's a question for you: what predictions do current MOND hypotheses make that match a wide range of observations?

I don't see how MOND can be reconciled with the fact that galaxies seem to have different amounts of dark matter (or variance from predicted gravity, if you prefer not to speak of DM) in varying distributions. If you could show me that, it'd move me a bit away from the dark (matter) side.

[turbo-1]

There's a question for you: what predictions do current MOND hypotheses make that match a wide range of observations?

I don't see how MOND can be reconciled with the fact that galaxies seem to have different amounts of dark matter (or variance from predicted gravity, if you prefer not to speak of DM) in varying distributions. If you could show me that, it'd move me a bit away from the dark (matter) side.

I don't think MOND is the answer, nor do I believe that it has any wider applicability beyond galactic rotation curves. Its principal value has been in opening the discussion regarding gravitation and quantifying the failure of the standard model to predict the behavior of galactic masses. The mainstream assumes that Newton got gravity right, and that Einstein refined that, so that our understanding of gravity is perfect and unassailable. Given this level of faith, they had to find some way to explain the broad rift between expectation and observation, and they invented DM to solve the problem. If DM is distributed "just so" it can fix the galactic rotation curve problem, if it is distributed "just so" it can explain the excess gravitational binding of clusters, and if it is distributed "just so", it can account for the apparent walls and filaments of large-scale structure. To borrow Nereid's expression, that's a big steaming heap of invisible pink fairies.

How do we get out of this mess? First of all, we go back to what we know. The Standard Model (prior to the addition of the DM epicycle) is not predictive on galactic scales or larger. Gravity in these domains does not follow the simple rules that work so well for simple systems like our solar system. Instead of inventing DM for which there is NO evidence at this time, we must find out why Newtonian gravitation and GR fail on large scales. I believe the key to the problem is that gravitation is dynamic, and the gravitational field is polarized in domains populated by lots of matter, resulting in fields of greater intensity within very massive domains. Gravitation is not a simple kinematic exercise definable by the masses and separations of the matter involved - there is also the matter of field strength.

Until the end of his life, Einstein was looking for a dynamical ether to fix the shortcomings of GR. This ether would not be a fixed thing, but would vary in in its qualities in accordance with the masses embedded in it. He insisted that gravitational attraction, inertia, and centrifugal effects all arise from matter's interaction with the ether. Einstein also insisted that the ether was the transmissive medium by which EM traverses "empty" space. In the mid-1960s Sakharov hypothesised that gravitation and inertia arise from matter's interaction with the quantum vacuum field. This is not a crazy idea, because if quantum theory is correct (and it has a pretty good track record) the vacuum contains the bulk of the mass/energy in the Universe.

I don't want to hijack the MOND discussion with my views on gravitation, but needed to point out that we don't understand gravitation sufficiently well to justify the invention of DM. MOND has quantified the failure of the standard model's gravitation and has made accurate predictions. This tells us that the failure of the mainstream view of gravity is consistent and perhaps understandable, with a bit of work. In this sense, MOND has been one of the most useful exercises in decades. It is never going to explain cluster binding or large-scale structure, but it is pointing us to a more general formulation of gravity that will be able to do so.

[snarkophilus]

MOND has quantified the failure of the standard model's gravitation and has made accurate predictions.

Okay, that's what I was asking for... what predictions? I've heard LSB observations mentioned somewhere before -- how do these match to MOND? Are they uniformly supportive of it, or are there contradictory results as well?

Besides, MOND isn't really necessary to quantify the difference between the SM and observation. One can just look at where things are and where they ought to be.

[turbo-1]

Okay, that's what I was asking for... what predictions? I've heard LSB observations mentioned somewhere before -- how do these match to MOND? Are they uniformly supportive of it, or are there contradictory results as well?

Yes, MOND predicted the rotation curves of LSB galaxies a decade before they were measured. I don't know if these results have been uniformly accepted.

Besides, MOND isn't really necessary to quantify the difference between the SM and observation. One can just look at where things are and where they ought to be.

You're right, of course, but the fact that MOND was proposed is a watershed. It shows that at least some people are resistant to tacking epicycle after epicycle onto an already bloated model. Also, the fact that MOND can apply some simple rules and undo the failure of the SM to predict the rotation curves of galaxies is encouraging, because it suggests that correcting our gravitational model is not going to be too daunting.

[RussT]

I don't think MOND is the answer, nor do I believe that it has any wider applicability beyond galactic rotation curves. Its principal value has been in opening the discussion regarding gravitation and quantifying the failure of the standard model to predict the behavior of galactic masses. The mainstream assumes that Newton got gravity right, and that Einstein refined that, so that our understanding of gravity is perfect and unassailable. Given this level of faith, they had to find some way to explain the broad rift between expectation and observation, and they invented DM to solve the problem. If DM is distributed "just so" it can fix the galactic rotation curve problem, if it is distributed "just so" it can explain the excess gravitational binding of clusters, and if it is distributed "just so", it can account for the apparent walls and filaments of large-scale structure. To borrow Nereid's expression, that's a big steaming heap of invisible pink fairies.

How do we get out of this mess? First of all, we go back to what we know. The Standard Model (prior to the addition of the DM epicycle) is not predictive on galactic scales or larger. Gravity in these domains does not follow the simple rules that work so well for simple systems like our solar system. Instead of inventing DM for which there is NO evidence at this time, we must find out why Newtonian gravitation and GR fail on large scales. I believe the key to the problem is that gravitation is dynamic, and the gravitational field is polarized in domains populated by lots of matter, resulting in fields of greater intensity within very massive domains. Gravitation is not a simple kinematic exercise definable by the masses and separations of the matter involved - there is also the matter of field strength.

Until the end of his life, Einstein was looking for a dynamical ether to fix the shortcomings of GR. This ether would not be a fixed thing, but would vary in in its qualities in accordance with the masses embedded in it. He insisted that gravitational attraction, inertia, and centrifugal effects all arise from matter's interaction with the ether. Einstein also insisted that the ether was the transmissive medium by which EM traverses "empty" space. In the mid-1960s Sakharov hypothesised that gravitation and inertia arise from matter's interaction with the quantum vacuum field. This is not a crazy idea, because if quantum theory is correct (and it has a pretty good track record) the vacuum contains the bulk of the mass/energy in the Universe.

I don't want to hijack the MOND discussion with my views on gravitation, but needed to point out that we don't understand gravitation sufficiently well to justify the invention of DM. MOND has quantified the failure of the standard model's gravitation and has made accurate predictions. This tells us that the failure of the mainstream view of gravity is consistent and perhaps understandable, with a bit of work. In this sense, MOND has been one of the most useful exercises in decades. It is never going to explain cluster binding or large-scale structure, but it is pointing us to a more general formulation of gravity that will be able to do so.

Turbo-1, whether you call it DM + DE, aether, quintesence, quantum vacuum field, or mass/energy field, or stress-energy tensor, as Celestial Mechanic did to try to explain how it works, and how it 'anchors' (they are anchored, but this does not explain it) galaxies to the space they are in, it is all the same thing...space/spacetime, and it is made of something physical, it is not a pure vacuum.

Since it is impossible to escape the necessity for 'extra gravity' (DM) to explain cluster dynamics, it is more than likely necessary for galaxy rotation curve a well.

However, Dark Energy is an entirely different story!!!

Clusters and galaxies are not universe theory specific!

[Exeter]

I hope all will forgive me for not being a professional cosmologist but I wanted to throw my own two cents in. I tend towards the MOND view myself for the usual reasons. I think at heart that the scientists sense that if MOND is actually the root of the disease that that necessarily means that GR must be removed to cure the patient, and no one wants to be told that both their legs must be cut off to save them.

In my poor amateur view the salient thing that I notice is a steady progressive disparity of expected mass with increase of scale. This seems to suggest to me a regular progressive increase in gravitational force with distance. But how to explain this in a simple way? Newton states that the strength of the force decreases according to 1/d^2. Observation shows that the exponent of 2 must gradually decrease with scale. For galaxies the exponent n seems to be about 1 and decreases further at larger scales. From pure geometry we know the exponent should be 2 but it doesn't cooperate with what we want. There is a hidden assumption here about force propagation that we seem to be ignoring, it is that the force is propagating at a CONSTANT speed. Let us for a moment give this notion up and consider the possibility that gravitation is propagated by gravitons that after they are emitted from the gravitating body constantly accelerate. This changes the force dynamic now because at greater distances the gravitons will impart more energy per solid angle than we expect. Where does the energy come from to accelerate the gravitons? I'm not sure, from the zero point energy background of space or perhaps photons as they travel through space. That might account for progressive redshift with distance.

Regardless of whether I am on the right track or not I feel the basic problem is that the scientists feel they have a good understanding of gravity when in fact they know next to nothing about it. They cannot even say with authority what speed it propagates at other than theoretical conjecture. With such ignorance it is an oddity to me that they lock themselves tenaciously into certain approaches.

[dgruss23]

Since it is impossible to escape the necessity for 'extra gravity' (DM) to explain cluster dynamics, it is more than likely necessary for galaxy rotation curve a well.

I wouldn't say "impossible" for galaxy clusters. MOND reduces the mass discrepancy in clusters from ~10 to ~2-3. There is a possibility that this mass may yet be discovered in baryonic form. But we'll have to see. Another possibility would involve incorrect mass estimates if cluster distances are incorrect or if cluster velocity dispersions are contaminated by factors better discussed in Arp et al.

Even if DM is the ultimate answer to galaxy rotation curves, it must be explained why that DM is behaving in a MOND-like fashion.

[dgruss23]

Okay, that's what I was asking for... what predictions? I've heard LSB observations mentioned somewhere before -- how do these match to MOND? Are they uniformly supportive of it, or are there contradictory results as well?

I don't have time right now, but I'll try to get back to you on this in the next day or 2. For the moment let me just say that Milgrom made predictions about LSB galaxies and MOND before data was available actually test those predictions. When the data became available MOND's predictions were confirmed.

McGaugh has become a big player in the MOND discussion. If you read his MOND pages he describes (at least if I'm remembering correctly that is where he shares this) how he was wrestling with inconsistencies between his data for galaxies and the predictions of CDM. Then he saw Milgrom speak and the problems he was wrestling with were being explained by Milgrom in the context of MOND.

[dgruss23]

Well, no... they say it does interact with baryonic matter. Just not through photons, but through gravity instead.

And that is ultimately the problem with CDM. The observations of galaxies reveal that DM behaves in ways inconsistent with collisionless CDM.

I'm undecided, and awaiting the next installment in Nereid's thread in the Q&A section. I'm not convinced for or against at the moment, but it appears that there is some very strong observational evidence for it. I'm the sort of person who is predisposed to a MOND-like solution to the problems that dark matter tries to solve, but there's a lot of evidence for dark matter. If I can be shown that DM theory allows us to make predictions, I'll be close to sold on it.

It will be interesting to see what Nereid comes up with because there is quite a body of literature that puts CDM in serious trouble. The successes of CDM RE large scale structure are mirrored by significant failures on galaxy scales.

There's a question for you: what predictions do current MOND hypotheses make that match a wide range of observations?

I don't see how MOND can be reconciled with the fact that galaxies seem to have different amounts of dark matter (or variance from predicted gravity, if you prefer not to speak of DM) in varying distributions. If you could show me that, it'd move me a bit away from the dark (matter) side.

I'll have to take a stab at this one too - but not tonight.

[RussT]

I wouldn't say "impossible" for galaxy clusters. MOND reduces the mass discrepancy in clusters from ~10 to ~2-3. There is a possibility that this mass may yet be discovered in baryonic form. But we'll have to see. Another possibility would involve incorrect mass estimates if cluster distances are incorrect or if cluster velocity dispersions are contaminated by factors better discussed in Arp et al.

Even if DM is the ultimate answer to galaxy rotation curves, it must be explained why that DM is behaving in a MOND-like fashion.

dgruss23, above you say MOND reduces the discepancy from ~10 to ~2-3, however, below shows a totally different picture.

Nereid Wrote; From her Cold DM thread
[The good news is that we can constrain these uncertainties quite well, so while our estimates of the total mass of the stars in a galaxy may be off by ± 20% or so - averaged over all the bright galaxies in a rich cluster - the answer is still the same ... stars in the galaxies in rich clusters make up no more than a few, typically around 1 or 2, percent of the total cluster mass!

Just to finish this post - the gas and dust content of galaxies can be estimated using a variety of techniques, and for bright galaxies in rich clusters it turns out these constituents are quite minor - there's less mass in the gas and dust in galaxies in rich clusters than there is in the stars in those galaxies.]

If you just do a slight correction for some more baryonic matter, this would come up to 4% baryonic matter for the cluster...sound familiar?

But, dgruss23, how can it be that your example and Nereids are so different?

[Cougar]

The observations of galaxies reveal that DM behaves in ways inconsistent with collisionless CDM.

What observations are those? DM was originally put forward because the observations of galaxies showed that the typical rotation dynamics were quite different than the expected keplerian orbits. It was as if the galaxy had much more mass distributed more or less evenly throughout the galaxy and extending beyond the visible edge, hence the supposition of DM. In any case, the dark matter idea was derived from observations of galaxies.

In "variable gravity" theories, one might naturally imagine that the effect of gravity could conceivably lose some strength as it crosses many, many lightyears. Maybe some gravitons get scattered or something. BUT this is just the opposite of what is needed to explain the observations of galaxy dynamics and cluster dynamics, which require MORE gravity as distance increases. How is Nature going to pull that off?

[dgruss23]

dgruss23, above you say MOND reduces the discepancy from ~10 to ~2-3, however, below shows a totally different picture.

If you just do a slight correction for some more baryonic matter, this would come up to 4% baryonic matter for the cluster...sound familiar?

But, dgruss23, how can it be that your example and Nereids are so different?

Nereid is referring to estimates from Standard Newtonian Dynamics whereas I'm talking about estimates from Modified Newtonian dynamics. They make different predictions. The observed mass is the same in either standard or modified dynamics. What is different is that the observed dynamics of the clusters result in very different estimates of the total mass of the cluster. With standard dynamics the dynamical mass is about 10x the observed mass. With MOND, the dynamical mass is only ~2-3x the observed mass.

See Figs 1 and 2 of Sanders (1999). Actually in that analysis He notes that the mass discrepancy is reduced from 4 to 2 when going from standard dynamics to MOND dynamics. The variations in estimate may also depend upon the size adopted. Sanders used the inner 1 Mpc around the core whereas with a larger radius you would get a higher mass discrepancy - at least in Newtonian dynamics.

[dgruss23]

What observations are those? DM was originally put forward because the observations of galaxies showed that the typical rotation dynamics were quite different than the expected keplerian orbits. It was as if the galaxy had much more mass distributed more or less evenly throughout the galaxy and extending beyond the visible edge, hence the supposition of DM. In any case, the dark matter idea was derived from observations of galaxies.

Sure observations of galaxies were interpreted as requiring DM - where DM is generic for DM of any form -- baryonic or non-baryonic and all the various types of non-baryonic.

But researchers favor Cold Dark Matter (CDM) specifically. CDM is collisionless which means it only interacts with baryonic matter gravitationally (no influence from collisions). CDM makes very specific predictions about individual galaxies that are inconsistent with actual observations of galaxies - observations that are hard to explain if DM is collisionless CDM. As I said:

And that is ultimately the problem with CDM. The observations of galaxies reveal that DM behaves in ways inconsistent with collisionless CDM.

The fact that DM was inferred from observations of galaxies does not mean that every proposed DM candidate is consistent with observations of galaxies.

In "variable gravity" theories, one might naturally imagine that the effect of gravity could conceivably lose some strength as it crosses many, many lightyears. Maybe some gravitons get scattered or something. BUT this is just the opposite of what is needed to explain the observations of galaxy dynamics and cluster dynamics, which require MORE gravity as distance increases. How is Nature going to pull that off?

As noted before MOND is first and foremost a phenomenology that makes predictions about the dynamical behavior in low acceleration regimes. It does not provide an explanation in terms of gravitons. It is simply modified Newton's Law.

I can provide references to keep you busy for a very long time on all this (MOND and CDM difficulties as separate issues). But I'm giving Nereid a chance to finish her thread so that I've seen the case she wishes to make in its entirety. I'm also waiting for her answers to the questions I asked on this thread.

[Exeter]

In "variable gravity" theories, one might naturally imagine that the effect of gravity could conceivably lose some strength as it crosses many, many lightyears. Maybe some gravitons get scattered or something. BUT this is just the opposite of what is needed to explain the observations of galaxy dynamics and cluster dynamics, which require MORE gravity as distance increases. How is Nature going to pull that off?

I believe I made a suggestion for that problem in my previous post. If gravitons accelerate after they leave the body then gravitation will have excessive strength at larger distances.

[snarkophilus]

I believe I made a suggestion for that problem in my previous post. If gravitons accelerate after they leave the body then gravitation will have excessive strength at larger distances.

I'm not certain why this might be. You seem to be assuming a link between the speed of a graviton and the amount of force it imparts, but I see no reason why this should be the case (that's assuming that there really are gravitons, though they have yet to be observed).

In any case, gravitons, if they do exist, need to be massless, so they must move at the speed of light, and thus can't accelerate. Whether there's a Doppler shift-like effect associated with them is maybe an interesting idea, but I'm willing to bet that someone's investigated that already.

Anyone here know enough to say whether or not it's possible (in any theoretical framework that includes them or similar particles/strings/whatever) for gravitons to come with different energies, like photons? Could one have gravitons of different wavelengths, and a relation similar to the photon's E=hv?

[turbo-1]

The concept of gravitons as mediating particles for the gravitational field has one very big logical flaw. It is assumed that matter derives its mass from interaction with the Higgs field and its carrier particle - the Higgs boson. If matter derives its gravitational attraction from interaction with the gravitational field and its carrier particle (the graviton), how do we explain the fact that gravity seems to operate according to the same rules everywhere we look? Fields are not static things. They are polarized by interaction and can evolve. For gravity to act the same everywhere we look, the gravitational field and the Higgs field would have to be congruent everywhere and everywhen. Shall we believe in such a remarkable coincidence, or should we consider that mass and gravitational attraction arise from matter's interaction with a single field?

[firstcontact]

For gravity to act the same everywhere we look, the gravitational field and the Higgs field would have to be congruent everywhere and everywhen. Shall we believe in such a remarkable coincidence, or should we consider that mass and gravitational attraction arise from matter's interaction with a single field?

Wait a minute back up it's already known that gravity [I]does not[I]behave the same everywhere we look namely on large scales...

[Cougar]

Wait a minute back up it's already known that gravity does notbehave the same everywhere we look namely on large scales...

No. Most astro-people think gravity consistently behaves according to General Relativity, regardless of scale. That's why when viewing galaxy and cluster dynamics, it is concluded, rightly or wrongly, "Well, there must be more mass out there that we aren't detecting."

[firstcontact]

So you believe in DM which is fine

[Exeter]

I'm not certain why this might be. You seem to be assuming a link between the speed of a graviton and the amount of force it imparts, but I see no reason why this should be the case (that's assuming that there really are gravitons, though they have yet to be observed).

If the graviton is accelerating then the flux of energy per solid angle must increase at greater distances. What is changing here is the time relation. If the power at r=1 is 1 then at constant speed at r=2 the surface area will be 4 times greater and the power per solid angle is now 1/4. However, if gravity was to double in velocity in the interim then energy per unit time is doubled and the felt effect would be 2*1/4=1/2.

In any case, gravitons, if they do exist, need to be massless, so they must move at the speed of light, and thus can't accelerate. Whether there's a Doppler shift-like effect associated with them is maybe an interesting idea, but I'm willing to bet that someone's investigated that already.

Naturally I am assuming that relativity incorrectly applies to the superluminal realm. It necessarily has to because any superluminal phenomena automatically disproves it. When I say "graviton" I mean the term in the most generic possible interpretation i.e. a particle that mediates gravitation. I do not mean a particle that necessarily has the typical baggage of properties that relativists force it to have. In any case, I am envisioning a photon-like particle. The photon can change frequency but not speed. My graviton is similar but inverted in that it can change its speed but not its frequency. Thus my graviton accelerates when energy is added to it.

The concept of gravitons as mediating particles for the gravitational field has one very big logical flaw. It is assumed that matter derives its mass from interaction with the Higgs field and its carrier particle - the Higgs boson.

Assumed by you and theoretical physicists, not by me.

Fields are not static things. They are polarized by interaction and can evolve. For gravity to act the same everywhere we look, the gravitational field and the Higgs field would have to be congruent everywhere and everywhen. Shall we believe in such a remarkable coincidence, or should we consider that mass and gravitational attraction arise from matter's interaction with a single field?

Gravity does not act the same everywhere we look. Every time the gravitational constant is measured in mines or deep boreholes it measures higher than in the laboratory. There are numerous gravitational anomalies that have been found in experiments. It is even now admitted that getting a consistent value is extremely difficult. Most of these anomalies are dismissed away as though they aren't important.

[Cougar]

Gravity does not act the same everywhere we look. Every time the gravitational constant is measured in mines or deep boreholes it measures higher than in the laboratory.

Citation, please?

[Exeter]

Citation, please?

Albert T. Hsui
Science (ISSN 0036-8075) vol 237, Aug 21, 1987 p. 881-883
Borehole measurement of the Newtonian gravitational constant

Gravimetric measurements in a borehole within the Michigan Basin, obtained in September 1983, were utilized to estimate the Newtonian gravitational constant. Gravitational constants are computed using gravity measurements from two stations along the same vertical and by knowing the total rock mass sandwiched between these two stations. The calculation of rock formation density using a gamma-gamma density log is described. The gravity values are analyzed in terms of reference surface values, and it is observed that the GRAVITY INCREASES WITH DEPTH <emphasis mine>. Borehole measurement determined gravity constant values ranged from 6.6901 + or - 0.0668 x 10 to the -11th cu m/kg sec sq (at station separation 264.5 + or - 0.5 m) to 6.7000 + or - 0.0650 x 10 to the -11th cu m/kg sec sq (at 1163.5 + or - 0.5 m), which are higher than the laboratory value of Luther and Towler (1982) of 6.672 + or - 0.0004 x 10 to the -11th cu m/kg sec sq. It is noted that the data correlate well with the values of Stacey (1981).

(Eckhardt, D.H., et al; "Experimental Evidence for a Violation of Newton's Inverse-Square Law of Gravitation," Eos, 69:1046, 1988.)

"We have performed an experimental test of Newton's inverse-square law of gravitation. The test compared accurately measured gravity values along the 600 m WTVD tower near Raleigh, North Carolina, with upward, continued gravity estimates calculated from ground measurements. We found a significant departure from the inverse-square law, asymptotically approaching -547 ± 36 microGal at the top of the tower. If this departure is derived from a scalar Yukawa potential, the coupling parameter is alpha = 0.023, the range is lambda = 280 m, and the Newtonian Gravitational Constant is G = (6.52 ± 0.01) x 1011 m3 kg-1 s-2. We do not yet have adequate resolution to discriminate this scalar model from a scalarvector model."

(Zumberge, Mark Al, et al; "Results from the 1987 Greenland G Experiment," Eos, 69:1046, 1988.)

"In the late summer of 1987, an experiment was performed to determine the value of the Newtonian gravitational constant, G, by measuring the variation of the earth's gravity, g, with depth in the Greenland icesheet. The site for the experiment - the radar station at Dye-3, Greenland - was selected because of the existing 2000- m-deep ice borehole there. Previous analysis of ice-cores from the borehole indicate that the ice density can be accurately modeled. Gravity measurements were made to a depth of 1673 meters in the ice, the sub-ice topography was mapped with high-precision radar echo sounding over a 10-km-diameter region, and a series of 24 locations in a 32-km-diameter network around the hole were surveyed with gravity, leveling, and GPS positioning.
"When corrected for the sub-ice topography, a gravity anomaly that accumulated to nearly 4 mGal in 1.4 km was observed. We find measured anomalies can be taken as evidence for non-Newtonian gravity, but can also be accounted for in terms of Newtonian physics if a suitable distribution of high densiity masses exist beneath the borehole."


The last experiment is the most interesting in that the Greenland ice sheet was specifically chosen to rule out mass anomalies and yet mass anomalies always seem to be used to get out of the problem. This phenomena has been measured enough times that it is even called the "borehole anomaly" in the literature. None of this is smoking gun yet but it hasn't gone away either.